The present invention relates to potting of tubes in a housing to achieve a seal between the outsides of the tubes. The seal separates the housing into discrete chambers, with the tubes extending through the seal and in both chambers. In particular, the present invention may be used in oxygenators, sealing tubes which transmit gas or a heat transfer fluid from blood which passes over the outside surface of the tubes. The present invention may also be used in pheresis devices which separate a component from blood, also known as "plasmapheresis" or "apheresis" devices, or in a wide variety of other medical and technical devices.
Many modern devices for filtration, mass transfer and heat transfer in medical and other industries utilize a hollow fiber construction for efficient, compact operation. The tubular fibers are made from various materials such as polyolefins, cellulose, polysulfone, silicon rubber, polypropylene and others. The walls of these fibers may be either permeable or impermeable to the fluids carried therethrough, depending on the desired effect of the fluid transferred through the fiber with the fluid transferred over the fiber.
In particular, oxygenators for mass transfer between gas and blood may include hollow fibers disposed in a housing. The oxygenator functionally replaces the lung, adding oxygen and extracting carbon dioxide to transform venous blood into arterial blood. The primarily use for oxygenators is during cardiopulmonary bypass surgery.
The oxygenator may pump blood through the lumens of the hollow fibers while gas is exposed over the external surface of the capillary membranes. More preferably, gas flows through the lumens of the hollow fibers while blood is pumped over the external surface of the capillary membranes. This arrangement not only utilizes the larger outer surface area of the capillary tubes as gas transfer interface instead of the luminal surface, it also promotes blood mixing in a manner which enhances oxygen transport.
The external blood flow arrangement is most effective when the blood flows at generally right angles to the hollow fiber. The flowing blood successively encounters different fibers, and the transport of oxygen averaged over the periphery of each fiber is higher than with a parallel flow of blood. One structure to achieve a perpendicular external blood flow includes a blood inlet surrounded by gas flow fibers, with blood flowing radially outward through the fibers.
Blood pheresis, plasmapheresis or apheresis devices separate a specific component, such as plasma, a plasma component, white cells, platelets or red cells, from the remainder of the blood. Some medical procedures may include both "plasma exchange", in which the plasma is separated from the cellular components of the blood, and "plasma perfusion", in which the plasma is treated in a second filtration step to remove one or more specific components (such as a specific antibody, immune complex, globulin, toxin, protein, etc.) from the plasma. The treated blood may then be returned to the patient, often in combination with some type of replacement fluid.
Pheresis devices may use a hollow fiber construction to filter the blood as desired. Most commonly, any replacement component is allowed to mix into the blood during circulation through the patient. However, devices having a hollow fiber construction may also be used in some instances to place a replacement component back into the blood.
Oxygenators and/or pheresis devices must not induce trauma into the blood. The blood pressure and blood pressure differentials must not be too high, and the blood must be handled gently. The gas transfer membrane in an oxygenator must not allow the separation of plasma from the cellular components of the blood, which may cause plasma to seep through the fiber wall under pressure leading to a catastrophic decrease in membrane permeability. In contrast, the membrane in a pheresis unit may be intended to separate plasma from the cellular components of the blood.
In many applications, the blood must be maintained within a narrow temperature range, near body temperature. Often the surgeon/perfusionist may want careful control of the blood temperature, to "cool down" or "warm up" body functions of the patient. Oxygenators and/or pheresis units are therefore commonly coupled with heat transfer mechanisms to control blood temperature. For instance, heat transfer tubes having a controllable temperature may extend across the blood flow. One design includes using a heat transfer fluid such as water to transfer heat to and/or from the tubes. Similar to the gas exchange, heat transfer fluid may be channeled through hollow tubes which extend perpendicular to the blood flow, with blood flowing over the external surface of the heat transfer tubes. The tube walls for the heat transfer fluid should be thin and have a high heat conductivity, but should be impermeable to the heat transfer fluid and to blood.
Pheresis units and oxygenators in particular must be very reliable against leakage. Leakage of blood into the gas flow, into the heat transfer fluid flow, or into the separated blood component flow can clog the gas, heat transfer fluid or separated blood component flow and be detrimental to system performance, but does not immediately compromise the safety of the patient. Of more importance, direct flow of either oxygen (such as bubbles) or heat transfer fluid into the blood can catastrophically affect the patient. Leakage of water, for instance, into the blood can create severe hemolysis. The design of the oxygenator or pheresis device preferably allows complete, reliable separation between the blood flow path, the gas or separated component flow path, and (if present) the heat transfer fluid flow path.
The hollow fiber membranes in many current oxygenators are formed of microporous polypropylene. The fiber walls may have a wall thickness of 25 to 60 microns with a nominal pore size such as around 0.1 microns. With this wall thickness and pore size, the fiber walls are permeable to oxygen and carbon dioxide to allow free gas diffusion with the blood. The polypropylene is hydrophobic, and the fiber walls have a high enough surface tension to prevent plasma filtration at the moderate pressures in oxygenators.
Oxygenators must be competitively priced. Oxygenators must also be sterilized for each use. Because current oxygenator designs are not easily sterilized after use, oxygenators are disposable single-use items, which heightens the importance of cost. Pheresis devices and other medical fluid treatment devices have similar concerns. Microporous luminal fibers can be fabricated out of polypropylene on large scale and in defect free condition at a reasonable cost.
To form the blood treatment device, it is necessary to organize, hold and seal the luminal paths from the external blood flow path. One way to organize the hollow fibers is to weave the fibers into a mesh fabric, with the blood flow directed through the mesh fabric layer.
Seals are formed at the extremities of the hollow fibers, so the blood flow cannot come into direct contact with the luminal flow. For instance, the fibers may be "potted" in a sealing material to seal between the external surface of all of the fibers and the oxygenator housing. Typical potting compounds include acrylics, polyurethanes or epoxies.
Prior to placement of the potting compound, the bores of the fibers may be sealed or plugged in some manner to prevent potting compound entry. Luminal plugging may be performed by a heat or sonic seal. Luminal plugging may also be performed by a preliminary potting step, wherein the hollow fibers are potted to a shallow depth to form "potting caps" which plug the ends of the fibers. Potting caps are subject to leakage upon exposure to the primary potting compound, making controlling potting depth more difficult. The luminal plugging depth should be significantly less than the depth of the primary potting.
After the seals are formed between the hollow fibers, the hollow fibers are then manifolded or otherwise opened at their ends for luminal flow therethrough. For instance, after the potting compound has cured or otherwise solidified, a cutting operation may be used to cut off the plugged ends of the hollow fibers and a portion of the potting compound. The cutting typically requires very sharp cutters, usually with a lubricant of some type. The cutting operation exposes the inner bores of the fiber. The cutting process generates heat and chips which must be removed from the fibers and the housing. Excessive heat from cutting may reclose the lumen by smearing the potting compound over the cut surface.
In the typical potting and cutting operation, the manifolds and ports for the oxygenator are not added until after the cutting operation. Potting can occur directly in the housing, but then very special cutters and techniques must be used to cut both the hard material of the housing and the potted fibers. After the cutting operation, separate caps or manifolds must be added to the housing to provide entry and exit to the inner bores of the fibers. An additional seal must be made for the caps or manifolds, allowing for additional quality rejects or field failures.
As with any type of seal, the possibility of a leak occurring along the external surface of one or more fibers is a problem with potting. Leaks may occur due to poor fiber treatment, poor cleaning, air bubbles, or a defect in the potting compound. To minimize chances of leakage, the potting operation may be performed using a centrifuge, with centrifugal force pushing the potting compound outward to the ends of the fibers. Centrifugal potting helps to limit wicking of the potting compound along the fibers, eliminates bubbles, and more tightly packs the potting compound around the fibers.